style.9: update C99 commentary

Make style.9 read as a current statement of C99 preferences, rather than a
description of ongoing changes to our preferred style. Alsu use the short
form "ISO C99" on the 2nd and later instances rather than repeating the
unwieldy ISO/IEC 9899:1999 ("ISO C99") each time.

Reviewed by: cem, imp, jhb, kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23648